This invention relates generally to the control of electromechanical energy conversion and, more particularly, to brushless DC motors wherein commutation, speed, and/or position are controlled digitally.
Conventional DC motors employ sets of windings which are commutated during relative motion between the moving and stationary parts of the motor. This is true in both rotary and linear motors. Commutation may be accomplished either mechanically, by a commutator and brush assembly, or electronically, by means of solid state switching elements.
Commutation of the windings depends on the relative positions of the moving and stationary parts of the motor. In a brushless DC motor, comprising a rotor which rotates within a stator, the position of the rotor relative to the stator may be sensed optically or electromagnetically, for example by means of Hall effect sensors secured to the stator and positioned about a magnetized disc mounted on the rotor shaft.
Switching current in the motor windings may be accomplished by a bridge circuit. One such bridge circuit operated in conjunction with Hall sensors is shown in Malkiel U.S. Pat. No. 3,783,359 issued Jan. 1, 1974. The use of Hall sensors is also shown in Persson U.S. Pat. No. 4,086,519 issued Apr. 25, 1978. The bridge circuits normally use protective diodes with the transistors.
To avoid excessive power dissipation within the transistors, pulse width modulation of the current has been employed so that the transistors are either "ON" or "OFF". Pulse width modulation of a transistorized bridge switching circuit is disclosed in the aforementioned Malkiel patent.
Conventionally, the motor control circuit has been regarded as an entity separate from the motor itself, with separate control circuits being required to accomplish specific motor functions. In addition, separate motors have traditionally been employed to accomplish different functions. Thus, there are torque motors, servo motors, stepper motors, etc., each with its separate control circuitry. These practices, while conventional, are nonetheless expensive, inefficient and wasteful.